1. Field of the Invention
The present invention relates to a current terminal structure of superconducting wires and to a superconducting cable having this current terminal structure.
2. Description of the Related Art
In general, a superconductor has a structure in which a tape-like superconducting wire is wound around a core (former) in multi-layers. This thin film type superconducting wire to be used has a structure in which a superconducting layer formed of ReBCO (Re—Ba—Cu—O, wherein Re is a rare earth metal) for example is deposited, through a buffer layer, on a substrate, and a stabilizing layer formed of silver is deposited on the superconducting layer.
On passing an electric current to this superconductor, it is required to enable the electric current to uniformly flow through each of the superconducting layers of superconducting wires and also required to reduce the connecting resistance of current terminal portion so as to minimize the generation of heat at the connecting portion of the superconductor connected with a current lead for supplying an electric current.
The superconducting wire has a front side and a back side. The connection of the superconducting wire with the current lead at the current terminal is generally executed in such a manner that the superconducting layer side becomes outside or front side. If the superconducting wire is connected with the current lead so that the substrate side becomes outside, a large joule heat is permitted to generate due to a high current resistance of the substrate, thereby deteriorating the current efficiency.
When a magnetic field is applied perpendicular to the surface of a YBCO superconducting wire in which a YBCO (Y—Ba—Cu—O) layer is employed as the superconducting layer, a large AC loss is caused to generate. Namely, when the magnetic field becomes larger, the magnetic field is enabled to penetrate into the superconducting wire, thereby enabling Lorentz's force in the penetrating direction to balance with a pinning force. In the case of AC, since the magnetic field fluctuates periodically and hence moves against the pinning force, AC loss is caused to generate.
However, when a magnetic field is applied parallel to the surface of superconducting wire, the AC loss can be extremely minimized owing to the fact that the superconducting layer is as very thin as around 1 μm so that the region to which the magnetic field is able to penetrate is very thin. For example, when a cable is fabricated by using the YBCO superconducting wire, since a parallel magnetic field is mainly generated, the AC loss can be prominently minimized. However, since the width of actual superconducting wire is limited, a gap is caused to exist between wires in the cross-section of the cable, thereby forcing the magnetic field to be attracted into this gap. As a result, a perpendicular component of a magnetic field is caused to generate at this portion, thereby enabling most of the AC loss to be shared by this portion (see for example, N. Amemiya and Nakhata, Physica C 463-465 (2007) 775-780).
As described above, it is very important to reduce the perpendicular component of the magnetic field on applying the YBCO superconducting wire to electric power equipment.
Meanwhile, where a substrate exhibits magnetism, a magnetic field is attracted to the magnetic substrate, thereby making the magnetic fields depict complicated magnetic lines of force. Namely, a magnetic flux is caused to concentrate at an end of a wire, thereby increasing the vertical magnetic field at this end, resulting in an increase of current loss (see for example, N. Amemiya and Nakhata, Physica C 463-465 (2007) 775-780).
FIG. 7 illustrates a current terminal portion of an ordinary multilayered superconducting cable. In the example shown in FIG. 7, a first superconducting wire 22 constituting a first layer and a second superconducting wire 23 constituting a second layer are spirally wound around a former 21. In this case, the first superconducting wire 22 includes a substrate 22a and a superconducting layer 22b formed on the substrate 22a. Whereas, the second superconducting wire 23 includes a substrate 23a and a superconducting layer 23b formed on the substrate 23a. 
The first superconducting wire 22 and second superconducting wire 23 are both disposed in such a manner that the sides of the superconducting layer 22b and the superconducting layer 23b become outside. The ends of the first superconducting wire 22 and second superconducting wire 23 are bench-cut and integrated by a solder-fixing portion 24, thereby creating a current terminal portion (see for example, JP-A 2004-87265).
According to the current terminal portion constructed in this manner, it is possible to extremely minimize the connecting resistance and, furthermore since the magnitude of shunt to each of superconducting layers can be determined by the manner of winding spiral of the superconducting layers, it is possible to realize uniform shunt through the adjustment of this winding spiral.
FIG. 8 illustrates a current terminal portion of a multilayer superconducting cable having a magnetic substrate. In the example shown in FIG. 8, a first superconducting wire 22 constituting a first layer and a second superconducting wire 23 constituting a second layer are spirally wound around a former 21. In this case however, contrarily to the ordinary arrangement, the first superconducting wire 22 is disposed in such a manner that the side of the magnetic substrate 22a thereof becomes outside of the superconducting cable and the second superconducting wire 23 is disposed in such a manner that the side of the superconducting layer 23b becomes outside of the superconducting cable. In a conductor constructed in this manner, the influence of magnetism can be contained within the superconductor, thereby making it possible to minimize the AC loss (see for example, JP-A 2008-47519).
However, when the current terminal portion is constructed so that the first superconducting wire 22 is disposed so that the side of the superconducting layer 22b becomes inside, the connecting resistance would become larger, thereby making it impossible to enable the first and second superconducting wires to share the same magnitude of electric current. Namely, since the current terminal portion is not provided with a sufficient electrode capacity, joule heat is caused to generate at the moment when the electric current flowing through the second superconducting wire 23 exceeds a critical current Ic, resulting in a great increase in AC loss.